The present invention relates to an amplifier circuit for a two-wire lead ECM (Electret Condenser Microphone) and, more particularly, to an amplifier circuit using an operational amplifier.
A conventional amplifier circuit of this type using a single power supply is shown in FIG. 3. A bias voltage applied to a noninverting input terminal 10 of an operational amplifier 2 is generated by a bias circuit 8 constituted by voltage dividing resistors 4 and 5 and a capacitor 6 for reducing power source noise. Since a two-wire lead ECM 3 which applies its output to an inverting input terminal 14 of the operational amplifier 2 receives a power supply voltage and a signal through a common line, a power supply voltage is applied to the ECM 3 by a power supply circuit 9 constituted by resistors 11 and 12 and a capacitor 13. The resistor 12 serves as the load resistor of the power supply circuit 9, and the capacitor 13 reduces noise included in the power supply to prevent the S/N ratio of the ECM 3 from being decreased.
In addition, a capacitor 20 is used for cutting a DC component, and resistors 21 and 22 are used for setting the gain of the operational amplifier 2. Reference numeral 1 denotes an output terminal.
In the above conventional amplifier circuit, the bias circuit 8 for applying a power supply voltage to the noninverting input terminal of the operational amplifier 2 and the power supply circuit 9 for applying a voltage to the ECM 3 are required, and have independent circuit arrangements, respectively. For this reason, in portable equipment such as a portable telephone apparatus which is expected to be compact and lightweight, the number of parts is decreased. In the above conventional arrangement, however, the number of parts of the power supply system circuits 8 and 9 is increased. This is a serious limitation to a decrease in size of the portable equipment.